Numerical control devices have a reference point return function of returning a table or the like of a machine tool to a reference point, whereby the table or the like is manually moved toward the reference point after the power is turned on, and is decelerated in accordance with a signal from a deceleration limit switch. After the deceleration, the table or the like is then moved at a constant low speed (decelerated speed), and is stopped at a point in an electrical grid at which the deceleration limit switch loses contact with the deceleration dog and thus is closed.
FIG. 4 is a conceptual diagram of a conventional reference point return system. As shown in the figure, a deceleration dog 3 is fixed to a machine table 1, and the end of this deceleration dog 3 is set in such a manner that it is positioned between an electrical grid point GP2 corresponding to a reference point 4 and an electrical grid point GP1 on the left thereof. When the machine table 1 is moved in the direction shown by an arrow 1a, a deceleration limit switch 2, which is not in contact with the deceleration dog 3, is moved relative to the machine table 1 and comes into contact with the deceleration dog 3. The deceleration limit switch 2 is operated while it is in contact with the deceleration dog 3, i.e., for the length in the X axis direction of the deceleration dog 3, and outputs an off-state deceleration signal DEC.
As for a reference point return, the machine table 1 is moved in the direction shown by the arrow 1a at a rapid feed speed VH, by setting a mode select switch on an operator panel to a JOG mode, turning a reference point return switch ON, and pushing a feed button (+X) in the X axis direction. When the front end of the deceleration dog 3 reaches the deceleration limit switch 2, the deceleration signal DEC is turned off, and a deceleration of the machine table 1 is started. After the deceleration is ended, the machine table 1 is moved at a constant low speed VL after the deceleration, and the deceleration dog 3 is stopped at the electrical grid point GP2, after passing over the deceleration limit switch 2, to thereby end the reference point return.
Namely, a reference point return processing means of a numerical control device (CNC) reduces the moving speed VH of the machine table 1, and it is then moved at the constant low speed VL by a servomotor, not shown, in accordance with an output of the off-state deceleration signal DEC. When the deceleration dog 3 has passed the deceleration limit switch 2 during the movement at the constant low speed VL, the deceleration signal DEC is turned on. The reference point return processing means stops the machine table 1 at the first electrical grid point GP2 reached thereby after this deceleration signal DEC is made on. The reference point return is carried out by this series of operations. The deceleration limit switch 2 is provided with a break contact, and therefore, the on/off switching thereof is converse to the contact/non-contact thereof with the deceleration dog. Namely, while the deceleration dog 3 is passing over the deceleration limit switch 2, the deceleration signal DEC is made off.
The conventional reference point return system requires a deceleration dog having a length predetermined in accordance with the machine concerned, and this length must be set in accordance with the deceleration distance of the machine table.
Also, an adjustment is required so that the position at which the deceleration limit switch 2 is turned on, i.e., the position at which the trailing end of the deceleration dog 3 is located, is in the middle of the electrical grid points (between GP1 and GP2 in FIG. 4). This adjustment is extremely minute, because the pitch of the electrical grid is about several millimeters to 10 mm, and a problem arises in that a misadjustment can cause a deviation of one pitch.
Further, if the deceleration dog 3 is located under the machine table 1, for example, an external cover and the like must be removed to enable an accurate adjustment of the position of the deceleration dog, thus making the assembling and maintenance complicated.
No problem arises if the starting point of the reference point return (the position of the deceleration limit switch 2 relative to the machine table 1) is sufficiently far from the deceleration dog 3, such as R1, but if the starting point of the reference point return is located at the middle position R2 of the deceleration dog 3 or at the position R3 between the deceleration dog 3 and the reference point GP2, the operator must move the point from the positions R2 and R3.